A top U.S. diplomat in Pakistan said on Friday that the Obama administration does not know how many Americans might have disappeared overseas to train with al-Qaida or other terrorist groups, but the number is not thought to be large. Speaking to the Pacific Council on International Policy in Los Angeles, Ambassador to Pakistan Anne Patterson outlined a nightmare scenario'' in which people holding U.S. passports receive terrorist training then return legally to the U.S. to commit violent acts. They can easily infiltrate back into the United States and, frankly, we don't know what to do about them,'' Patterson said. We think there are more out there than we know about.'' We just have to keep working at it,'' she said. Ms Patterson said the U.S. is gathering information with Pakistan and other governments to identify and locate such people. It's not very many. But it's hard to get a precise number,'' Ms Patterson said. Earlier this month, prosecutors in Pakistan seeking to indict five Americans on terrorist charges submitted their case to a judge, accusing the men from the Washington, D.C., area of waging war against Pakistan and plotting to attack the country. The men were arrested in December in Punjab province not long after reaching Pakistan. They were reported missing by their families in November after one left behind a farewell video showing scenes of war and casualties and saying Muslims must be defended. A U.S. Senate report earlier this year said authorities believe as many as three dozen Americans who converted to Islam in prison have travelled to Yemen, possibly to train with al-Qaida. Some vanished for weeks at a time but still hold their U.S. passports.